fix(ttl) TTL issue in mutate function (Kong#56)

* feat(mutate) enhance mutate function
* refactor(revalidate) rename revalidate mutate
* fix(cache): fix issue when ttl was set to 0
* test(ttl): add ttl test cases

Fixes Kong#56 Kong#28

chore(release) 0.6.0

chore(release) 0.5.1

docs(readme) add entry to toc

chore(release) 0.4.0

docs(example) add vite readme

chore(*) release 0.3.2

chore(*) release 0.3.1

chore(*) release 0.3.0

feat(revalidation) add revalidation in mutate (Kong#17)

mutate was only warming the DATA_CACHE and was not triggering revalidations on
all swrv instances, so a user could not easily inject data via some async event
outside of the context of a useSWRV hook e.g. via websocket event. This adds all
the stateRefs into a cache (essentially a Map wrapper) that allows mutate to
access the vue reactivity when setting data/error/etc. Since each item in
REF_CACHE is reactive, setting their values will trigger rerenders in each
useSWRV instance.

This introduces a small change to existing functionality that if the fetcherFn
is a simple function that resolves immediately e.g. () => 'hello', then the data
 ref will never be undefined, but will be hello on first render.

Fixes Kong#16